Opens a web page in a browser. Requires relative or absolute url.
If url starts with `/`, opens a web page of a site defined in `url` config parameter.

```js
I.amOnPage('/'); // opens main page of website
I.amOnPage('https://github.com'); // opens github
I.amOnPage('/login'); // opens a login page
```

@param {string} url url path or global url.
@returns {void} automatically synchronized promise through #recorder
